Cellular phone systems permit the user of a mobile station to travel extensively and still retain access to cellular phone service. The mobile station is programmable and includes a number-assignment module (NAM) and an intelligent roaming database (IRDB). The NAM is programmable memory containing information about the mobile station such as electronic serial number, phone number, paging channels, etc. The IRDB contains information elements that the mobile station uses during scanning operations.
Advantageously, the user accesses a home service provider. The home service provider is the entity to which the user subscribes for service. Depending on the user's location, access to the home service provider may not be available or the signal may not be of sufficient strength. Additional service providers are categorized as partners, favored, neutral, or forbidden. A partner service provider is one which the home service provider has negotiated for superior subscriber services and rates. A favored service provider is similar to a partner except the service area of the favored service provider overlaps the service area of the home service provider. A neutral service provider is neither home, partner, favored, nor forbidden. In other words, there is no match with the IRDB in the mobile station or the NAM. A forbidden service provider is one that should never be used, with the exception of 911 service. The categories of service providers take precedence in the following order: home, partner, favored, neutral.
The mobile station prefers to camp on a home or partner service provider. These are known as "acceptable". A mobile station is also allowed to camp on a favored or neutral service provider. These are termed "unacceptable". When a mobile station is powered up, it immediately starts scanning for control channels to camp on. If the mobile station can only locate favored or neutral service providers, it will camp on one of these. When a mobile station camps on an unacceptable service provider, it periodically leaves that channel and searches for an acceptable service provider. The act of leaving a current control channel and scanning for others will degrade the paging performance. Particularly, the re-scanning procedure is made up of several steps which are repeated until an acceptable channel is found or all of the bands that the mobile station can scan are exhausted. When camping on a control channel, the mobile station must read 20 milliseconds of data from the current service provider every 1.28 seconds, defining a hyperframe. The operation of the station for the remaining 1.26 seconds is left up to the manufacturer. In general, the mobile station enters a low power state during this period when no data is needed.
Paging information from the current control channel arrives during the 20 millisecond time period. Once most mobile stations start the re-scanning procedure, they continue until the process has been completed. This process takes more than 1.26 seconds, causing the mobile station to miss pages. In fact, a reasonable time estimate for this procedure is on the order of 15 seconds to one minute. A phone is unable to receive pages while performing the re-scanning procedure in this manner.
The present invention is directed to overcoming one or more of the problems discussed above in a novel and simple manner.